The question I have is what is the units of the radius passed in? For example, if I want to draw a circle of raduis 10 miles, what value do I pass in? If I pass in 10, it just draws a really huge circle with radius several multiples of 10!

I tried to look into the sdk api docs and could not find much info, so any help will be appreciated.

That will use Meters (WebMercator's units). Note that your output may not be what you expect as the input or output approaches the poles. Also, there's no guarantee how many points the output polygon will have in its boundary.

Could be. Depends on what the projection is of your map. I'm guessing it's not Web Mercator :D

My solution will generate you a polygon that is pretty close to circular on the ground (i.e the edge is R miles north and R miles east of the center). If you are using a Web Mercator projection, it will appear circular on the map. If you are using WGS84 for example, it will indeed appear less tall as you move north or south from the equator, by virtue of the projection.

If you want something that looks circular on a WGS84 map, and you know that your center point is WGS84, you can either use your & Technobrat's solution or something like this:

But remember that this is approximating the conversion between decimal degrees and meters, which is not constant for longitude as latitude varies.

So, it really depends on what you are generating the 10 mile radius geometry for and (in short) how far north or south of the equator. Your solution may be good enough for what you want, but bear in mind that a degree of longitude will be very much less than 69 miles as you approach the poles. On the other hand, your application users may keep asking you why the "circle" is elliptical, and that might be a bigger headache than considering some things that might be more than 10 miles away.